The invention relates generally to dye transfer printers such as thermal printers, and in particular to the problem of crease or wrinkle formation in successive dye transfer areas of the donor web. Crease formation in the dye transfer area can result in an undesirable line artifact being printed on a dye receiver.
A typical multi-color dye donor web that is used in a thermal printer is substantially thin and has a repeating series of three different rectangular-shaped color sections or patches such as a yellow color section, a magenta color section and a cyan color section. Also, there may be a transparent colorless laminating section immediately after the cyan color section.
Each color section of the dye donor web consists of a dye transfer area that is used for dye transfer printing and a pair of opposite longitudinal edge areas alongside the dye transfer area which are not used for printing. The dye transfer area is about 95% of the web width and the two edge areas are each about 2.5% of the web width.
To make a multi-color image print using a thermal printer, a motorized donor take-up spool pulls the dye donor web from a donor supply spool in order to successively advance an unused single series of yellow, magenta and cyan color sections over a stationary bead of selectively heated resistive elements on a thermal print head between the two spools. Respective color dyes within the yellow, magenta and cyan color sections are successively heat-transferred via the bead of selectively heated resistive elements, in superimposed relation, onto a dye receiver such as a paper or transparency sheet or roll, to form the color image print. The bead of resistive elements extends across the entire width of a color section, i.e. across its dye transfer area and the two edge areas alongside the transfer area. However, only those resistive elements that contact the dye transfer area are selectively heated. Those resistive elements that contact the two edge areas are not heated. In other words, the dye transfer is effected from the dye transfer area to the receiver medium, but not from the two edge areas to the receiver medium.
As each color section, including its dye transfer area and the two edge areas alongside the transfer area, is advanced over the bead of selectively heated resistive elements, the color section is subjected to a longitudinal tension particularly by a pulling force of the motorized donor take-up spool. Since the dye transfer area is heated by the resistive elements, but the two edge areas alongside the transfer area are not, the transfer area is significantly weakened and vulnerable to stretching as compared to the edge areas. Consequently, the longitudinal tension will stretch the dye transfer area relative to the two edge areas. This stretching causes the dye transfer area to become thinner than the non-stretched edge areas, which in turn causes creases or wrinkles to develop in the transfer area, particularly in those regions of the transfer area that are close to the edge areas. The longitudinal creases or wrinkles are most notable in the regions of the dye transfer area that are close to the two edge areas because of the sharp, i.e. abrupt, transition between the weakened transfer area and the stronger edge areas.
As the donor web is advanced, the creases or wrinkles tend to spread or extend from a trailing or rear end portion of a used dye transfer area at least to a leading or front end portion of the next dye transfer area to be used. A problem that can result is that a crease or wrinkle in the leading or front end portion of the next dye transfer area to be used will cause an undesirable line artifact to be printed on a leading or front end portion of the dye receiver when dye transfer occurs at the crease. The line artifact printed on the dye receiver is relatively short, but quite visible.
The question presented therefore is how to solve the problem of the creases or wrinkles being created in an unused transfer area so that no line artifacts are printed on the dye receiver.
According to one aspect of the invention, there is provided a method of preventing crease formation in a dye transfer area of a dye donor web that can cause line artifacts to be printed on a dye receiver during a dye transfer from the dye transfer area to the dye receiver in a dye transfer printer. The method comprises:
heating the dye transfer area of the dye donor web sufficiently to effect a dye transfer from the dye transfer area to the dye receiver, and not heating opposite edge areas of the dye donor web alongside the dye transfer area sufficiently to effect a dye transfer from the edge areas to the dye receiver, but which therefore causes the dye transfer area to become more susceptible to being stretched than the edge areas; and
mechanically causing the edge areas to be stretched substantially the same as the dye transfer area, when the dye transfer area and edge areas are subjected to a pulling force that tends to stretch the dye transfer area and edge areas, to avoid a reduction in stretching from the dye transfer area to the edge areas that would form creases in the dye transfer area which can cause line artifacts to be printed on the receiver medium.
According to another aspect of the invention, there is provided a thermal printer capable preventing crease formation in a dye transfer area of a dye donor web that can cause line artifacts to be printed on a dye receiver during a dye transfer from the dye transfer area to the dye receiver. The thermal printer comprises:
a thermal print head adapted to heat the dye transfer area of the dye donor web sufficiently to effect a dye transfer from the dye transfer area to the dye receiver, and not heat opposite edge areas of the dye donor web alongside the dye transfer area sufficiently to effect a dye transfer from the edge areas to the dye receiver, but which therefore causes the dye transfer area to become more susceptible to being stretched than the edge areas; and
a platen roller that holds the dye transfer area and the edge areas against the thermal print head during the dye transfer from the dye transfer area to the dye receiver, and which is adapted to mechanically cause the edge areas to be stretched substantially the same as the dye transfer area, when the dye transfer area and edge areas are subjected to a pulling force that tends to stretch the dye transfer area and edge areas, to avoid a reduction in stretching from the dye transfer area to the edge areas that would form creases in the dye transfer area which can cause line artifacts to be printed on the receiver medium.